The desirability of broadband antenna systems operating in the microwave and millimeter wave frequency bands is by now indisputable. One of the most promising technologies for producing such systems for space-based communications is a phased array antenna which uses superconductors instead of normal metal for the network that distributes and controls the phase and amplitude of the signals to the radiating elements (the feed network). Superconductors have very low conduction loss when operated at temperatures below their superconducting transition temperature, T.sub.c.
This low distribution loss enables the use of a single or a relatively small number of transmit or receive amplifiers instead of having an amplifier associated with each radiating element in the antenna array. Superconducting delay lines can be used for the low-loss and wideband control of the phase of the radiated signals, providing good beam control over a wide band not possible with ordinary phase shifters. Low-loss superconducting switches have been demonstrated for producing switched delay modules. Superconducting flux-flow transistors have also been demonstrated which can perform phase and amplitude control.
As is quite often the case with scientific and engineering advances, however, the use of superconductors in satellite-based antenna systems brings in its own particular problems. The most critical in many applications is the necessity to keep the operating temperature of the superconductor materials well below the superconducting transition temperature. This requires a cooling system for the active elements.
The cooling system must meet stringent requirements for flight qualification. Of paramount importance are its size and weight. Once in orbit, the size and weight are insignificant over a wide range, but achieving orbit is critically affected by these considerations. In brief, cooling systems for antennas installed in satellites must be as small as possible. To achieve acceptable cooling power in a small enough package the cooling load must be reduced to a minimum. Thus, a space qualified cooling system will include not only a heat extractor, but also provisions for eliminating heat build-up in the first place. One such provision is a radiation shield that blocks infrared (IR) and visible light that can act to heat the antenna and feed structure. The requirements are not as stringent for ground-based systems, but for terrestrial applications the economic advantages of a reduced heat load and smaller cooling system may be even more important.
For practical applications it is crucial that the cooling system not interfere with the intended operation of the antenna. In the present case the implication is that the radiation shield must be transparent to the frequencies of interest for communications, or at least should not disturb the electric and magnetic field distributions in and near the antenna in a way that interferes with the antenna's operation.